This invention relates to stranded high electric current cable connectors for securing a coarse stranded cable to an object.
In a power distribution substation, it is common to have exposed cables with coarse strands (larger than 3 mm strand diameter, aluminium or 2 mm hard drawn copper) to provide for transmission of high continuous currents or transient fault currents.
An interface between the cable and the connector must therefore withstand continuous currents, which can present temperature problems, and must withstand transient fault currents which can create significant forces which increase in proportion to the square of the current.
Such an interface will ideally serve without deterioration for the life of the substation (40 years) must be reliable, removable, easy to apply, reusable and of lowest costs.
Various connection methods have been developed over the years and are presently in use, but with time and increased loading (higher currents) conventional connection methods have been found to be less robust than anticipated.
The most common connector for large stranded cables employs a bolted clamp type connection. The main deficiency of this connection method has been the loss, over time, of clamping pressure (relaxation) which is accelerated by thermal cycling. The pressure exerted on individual cable strands breaks the high resistance surface oxide at the cable/connector interface and strand to strand interface, due to material deformation. Thus, the higher the contact pressure the more oxide is destroyed resulting in a greater conductive area, resulting in an improved connection.
Another type of connector employs a permanently compressed (crimped) sleeve type connector. This is similar to the above described arrangement, however, the cable end is placed in a sleeve which is permanently crimped in place on the cable end. This achieves better exertion of pressure on the cable, though the exerted pressure is not omnidirectional and is limited in application area. Also, the connector is not reusable.
Another, less common connection method is achieved by welding an aluminum cable end into an aluminum connector. This results in a solid metallic connection between each cable strand and the connector. The disadvantage of this connector is that specialized welders are required to make the connection and again the connector is not reusable.
For bolted cable connections it appears the main mechanical cause of clamping relaxation is slow material deformation of the cable strands at pressure points between the strands. Then, a slow deformation of the clamp itself becomes a problem together with the bolts and their resting surfaces. Deformation is enhanced by airspace between the individual cable strands, which allow room for the conductor material to flow. Over time, the pressure points are enlarged and the relaxation decreases exponentially. This relaxation effect of the contact pressure between the connector takes place in most compressed cable connections, however, it is a slow process.
Another cause of clamping relaxation results from uneven forces created in a bolted clamp. As the pressure on the clamped cable end is not omnidirectional, the individual cable strands will migrate and become repositioned until an omnidirectional pressure is achieved. This process is faster than material deformation mentioned above and after only a few days results in a noticeable relaxation of clamping pressure.
Electrically caused losses of clamping pressure are created by uneven current sharing of the individual cable strands within the clamped space. Usually, only one half portion of a connector carries the bulk of the current so the strands pressing against an inside clamp surface of the clamp experience the highest current. This results in an elevated temperature which softens the strands allowing the strand material to flow into neighbouring airspace with the resultant mechanical relaxation. Thus, a connector might perform adequately for many years until a damaging current is reached.
Natural temperature cycling causes a contraction and expansion of the cable and connector materials. This, together with an electric current, accelerates relaxation, as current carrying micro contact areas (pressure points) within the clamp are opened, and then closed again, allowing high resistance oxides to form, increasing electrical resistance. This in turn creates a higher temperature and increased material flow leading to further contact pressure reduction, increased micro contact breakage and so on. This effect is known as "thermal runaway". Eventually, the clamping pressure on conventional connectors is no longer able to hold the cable in place during a fault, or the above thermal runaway condition catastrophically melts and burns the connection during a period of high current flow.
What would be desirable, therefore, is to have a simple, inexpensive, easy to install, reusable and robust connector that completely surrounds the cable end with a high uniform omnidirectional pressure and with a minimum electrical resistance.
The present invention addresses this need.